Hi. I googled your problem and came across this-
It could well be a hydraulic tappet, and once they've started making a noise they're finished. The additives you can buy will keep it quiet by expanding the oil seal but it won't last long. The best course of action is to try and identify the noisy one (use a screwdriver held against the cam cover and to your ear) and replace two or three. The M43 engine is quite easy to work on and when you have the special tool you can compress the valve and hook out the rockers before lifting out the old hydraulic tappet and dropping a new one in. Jim
The older I get the wrong bit gets stiff!!